Grazing animals exert pressure on the ground comparable to that of agricultural machinery. As a result, soil under pasture can be compacted. In grazing systems based on permanent pastures or rangelands, there is little opportunity to ameliorate poor soil physical conditions through tillage. Hence, it is important to understand the effects of grazing on soil physical properties and the consequent effects of these properties on pasture growth and composition. Most soils under grazed pasture, even those managed to minimise soil physical degradation, will be compacted to some extent. However, the magnitude of this compaction is usually small, and limited to the upper 50–150 mm of the soil. Compaction to greater depth, and other changes in soil physical properties, are more likely in recently tilled or wet soils. The response of pasture to the poorer soil conditions caused by grazing is difficult to determine, but it is likely to be small compared with the defoliation effects of grazing. Maintenance of a vigorous pasture should be a major aim of grazing management and would also achieve the secondary aim of maintaining acceptable soil physical conditions.

Grazing effects on soil physical properties and the consequences for pastures: a review

This chapter provides a comprehensive review of the literature relating to the impacts of grazing animals on the quality of soils, vegetation, and surface waters. It focuses on intensively managed grasslands where there is the greatest potential for these impacts to be observed. The chapter indicates that while well-managed grazing can be beneficial to the environment, intensively managed grazing can actually lead to the degradation of both the soil and vegetation of grassland environments. The various causes, forms, and consequences of this degradation are discussed in detail, and gaps in the knowledge are identified. The chapter highlights the need for recognition and quantification of the relationships between the on-site impacts of grazing animals (i.e., changes in soil properties and vegetation cover) and the off-site impacts of grazing animals (i.e., the impact of these changes on hydrology and water quality in surface waters), as these relationships have, in the past, only been alluded to by authors. However, there exists relatively little research evidence to support and quantify these relationships, thus herein we describe data required to address the lack of understanding of the role of grazing animals on grasslands. Finally, the last section of this chapter considers the land management and remediation options available for the reduction of the impacts of intensive livestock farming.

The impacts of grazing animals on the quality of soils, vegetation and surface waters in intensively managed grasslands

Models that integrate the influence of soil intrinsic attributes on the estimation of soil compaction are scarce for Hapludox. The present study tested the hypothesis that the compressive behavior of Hapludox with wide variations in intrinsic soil attributes can be estimated based on pedotransfer functions (PTFs). The general goal of this research was to determine the effect of intrinsic soil attributes on the susceptibility to compaction, preconsolidation pressure and compression curve of Hapludox, and to develop PTFs that allow the estimation of these parameters based on easily measurable soil attributes. The study was conducted on a soil toposequence that includes a sandy Typic Hapludox, a loamy Typic Hapludox, and a clayey Rhodic Hapludox. The uniaxial compression test was applied to 50 undisturbed soil samples at matric potential values of -10 and -100 kPa. After load withdrawal, soil bulk density, void ratio, gravimetric soil water content, particle-size distribution, particle density, and organic matter were determined. The compression curves, the compression index, and the preconsolidation pressure were obtained. The relationship between the compression index, soil bulk density, and clay content was statistically significant with R 2 = 0.77. Organic matter and soil water content did not affect the compression index. The preconsolidation pressure was significantly related with soil bulk density, soil water content, and clay content (R 2 = 0.70), but was unaffected by organic matter. Soil compressibility was dependent on soil bulk density. A nonlinear model fitted the data with R 2 = 0.90 allowing to predict the compressibility of soils for a wide range of stresses and inherent soil properties.

Susceptibility to compaction, load support capacity, and soil compressibility of Hapludox

Identifying the vulnerability of soils to compaction damage is becoming an increasingly important issue when planning and performing farming operations. Soil compaction models are efficient tools for predicting soil compaction due to agricultural field traffic. Most of these models require knowledge of the stress/strain relationship and of mechanical parameters and their variations as a function of different physical properties. Since soil compaction depends on the soil's water content, bulk density and texture, good understanding of the relations between them is essential to define suitable farming strategies according to climatic changes. In this work we propose a new pedotransfer function for 10 representative French soils collected from cultivated fields, a vineyard and forests. We investigate the relationship between soil mechanical properties, easily measurable soil properties, water content and bulk density. Confined compression tests were performed on remoulded soils of a large range of textures at different initial bulk densities and water contents. The use of remolded samples allowed us to examine a wide range of initial conditions with low measurement variability. Good linear regression was obtained between soil precompression stress, the compression index, initial water content, initial bulk density and soil texture. The higher the clay content, the higher the soil's capacity to bear greater stresses at higher initial water contents without severe compaction. Initial water content plays an important role in clayey and loamy soils. In contrast, for sandy soils, mechanical parameters were less dependent on initial water content but more related to initial bulk density. These pedotransfer functions are expected to hold for the soils of tilled surface layers, but further measurements on intact samples are needed to test their validity.

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A method for predicting soil susceptibility to the compaction of surface layers as a function of water content and bulk density.

When compressing soil, there is a characteristic relationship between compressive stress and volume change that can be used to define important soil mechanical properties. Two defining features can be determined – the compression index (Cc – the modulus of the slope of the linear virgin compression curve) and the precompression stress ( σ ′ p – the transition point between the elastic rebound curve and the virgin compression curve). These are indicators of compressibility and stress history respectively. The purpose of this paper is to evaluate different ways of estimating these indicators based on laboratory test data. Repacked soils with a range of textures were subjected to sequential compressions of 50, 100 and 200 kPa, which provided two compression characteristics with “known” σ ′ p of 50 and 100 kPa. Three functions were fitted to the measured test data (fourth-order polynomial, symmetrical logistic sigmoidal and asymmetrical Gompertz sigmoidal). Values of Cc were estimated by linear regression (for the data later fitted with a polynomial function) or by the tangent at the inflection point derived from model parameters (logistic and Gompertz functions). Three estimates of σ ′ p were calculated for each of the three functions: the standard Casagrande method (C), the intercept of the virgin compression curve and the initial (no stress) horizontal line (V–I), and the point of maximum curvature (MC) derived from the curvature function (κ). The accuracy of estimating σ ′ p and the magnitude of Cc generally increased with clay content. Estimates of Cc based on sigmoidal curves did not differ greatly from the linear regression estimate. Sigmoidal curves yielded σ ′ p estimates with lower absolute deviations from known values than polynomial-based estimates. The MC calculation based on the Gompertz function gave the most accurate estimate of σ ′ p . The lower asymptote of sigmoidal curves may also correspond to the water-filled pore space. Thus, despite the fact that all three functions fitted the measured data equally well, characteristics based on the sigmoidal curves were deemed to be most appropriate. The greater accuracy of the prediction of σ ′ p favoured the Gompertz function. Wider applicability of this was further checked with data selected from an independent database on subsoil compaction. We recommend fitting the Gompertz function to measured soil compression characteristic test data, and to define Cc objectively as the modulus of the slope of the tangent at the inflection point, providing this lies within the measured data range, and σ ′ p as the point of maximum curvature as defined by κ.

R. Earl

Papers

The Relationship among the Pre-compaction Stress, Volumetric Water Content and Initial Dry Bulk Density of Soil

The Effect of Injector Tine Design on Odour and Ammonia Emissions following Injection of Bio-solids into Arable Cropping

Assessment of the Behaviour of Field Soils during Compression

Development of a Technique for Assessing the Behaviour of Soil under Load

Prediction of Trafficability and Workability using Tensiometers